This application claims the priority of German patent document 100 18 193.7, filed Apr. 12, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for controlling an electric drive, in which a controlled variable (actual value) is adjusted to a desired value.
Basically, two types of control systems are known, specifically discontinuous and continuous control systems. Discontinuous control systems comprise, for example, two-position and three-position controllers. Continuous control systems comprise proportionally operating controllers (P-controllers), integrally operating controllers (I-controllers), proportionally-integrally operating controllers (PI-controllers) or derivative action controllers (PD- and PID-controllers).
Concerning the general prior art, reference is made to the textbook with the title xe2x80x9cIntroduction to Control Engineeringxe2x80x9d by Mann/Schiffelgen, 5th Edition, 1986.
However, in many applications, particularly in connection with electric drives, the actuator can become overloaded during constant and continuously implemented control. In electric drives, in particular, overheating can take place.
Such a thermal overstressing may occur, for example, during force transmission via a worm gear. When the sign preceding the load moment changes, the play inherent in the worms gear causes a backlash of teeth to be passed through. If the amplitude of this movement is sufficiently large that it is detected by an actual-value generator, a permanent control movement takes place which supplies energy for a change of sign preceding the moment to an actuating drive. In this manner, the actuating drive is overloaded on a short-term or medium-term basis.
In order to solve these problems, it has been suggested in the case of timed electric drives to guide the timing ratio toward zero when, in a first alternative, the cycle length itself is below a certain limit or when the deviation of the actual value from a desired value is within certain limits. By defining a timing ratio of zero, the controller is in fact switched off within a defined time period, during which the electric drive can recuperate.
However, both of the two methods have the disadvantage that either the desired value to be adjusted is not reached or that, when the controller is switched back on, breakaway forces are more or less taken into account. This is occurs, for example, when the controller is not switched back on before the pulse widths of a pulse-width modulation exceed a defined limit value.
One object of the present invention is to provide a control method which approaches a desired value as precisely as possible.
Another object of the invention is to provide such a control method which also ensures thermal protection of an electric drive.
These and other objects and advantages are achieved by the control method according to the invention, in which the desired value is approached (at least with respect to points) in an essentially precise manner in a first step. After the desired value has been reached, control operation is interrupted, so that no adjusting signal is supplied to the electric drive and continuous readjustment does not take place. The electric drive will be protected as a result of this interruption. In particular, thermal overheating can thereby be avoided.
During the interruption of the control operation, the difference between the controlled variable and the desired value is checked continuously to see whether it exceeds or reaches a defined amount, specifically a threshold. If so, the interruption of control operation is terminated and the controlled variable (actual value) is again adjusted to the defined desired value.
The method of operation according to the invention is particularly advantageous for electric drives with a slow drift of the actual value, which essentially corresponds to a relatively good self-locking. A field of application will therefore be, for example, the adjustment of a valve stroke in the case of a charge cycle valve of an internal-combustion engine by means of an electric drive. In such a drive, power is transmitted by way of a worm gear which has a relatively good self-locking property.
Proportionally operating controllers, integrally operating controllers or derivative controllers or controllers of a combination of the above-mentioned control properties can be used as the control devices. It is also possible to change the threshold dynamically, thus the distance from the desired value with respect to the amount. These dynamics can be geared to vehicle operating parameters, in which case the vehicle operating conditions have an influence on this threshold. According to a particularly simple embodiment, it is possible to find the threshold values in a characteristic curve or a characteristic diagram in which they are filed as a function of one or several operating values.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.